Elementary form that is very best gathered inside (7)
I believe the answer is:
isotope
'elementary form that is very best gathered inside' is the definition.
'isotope' can be an answer for 'elementary form'. I am not sure about the remainder of the definition.
'elementary form that is very best gathered' is the wordplay.
'elementary form' becomes 'ie' (I am not sure about this - if you are sure you should believe this answer much more).
'that is very' becomes 'so' (both can mean 'extremely').
'best' becomes 'top' (top can mean the very best).
'gathered' indicates putting letters inside.
'so'+'top'='sotop'
'ie' enclosing 'sotop' is 'ISOTOPE'.
Can you help me to learn more?
(Other definitions for isotope that I've seen before include "Distinct form of an element" , "O, is poet a species of atom?" , "One form of element, differing in its neutrons from another" , "Variant element form" , "O, is poet involved in chemical element?" .)